The present invention relates to the field of semiconductor based detectors for electromagnetic radiation. In particular active pixels for detecting electromagnetic radiation with a high fill factor or high sensitivity are disclosed as well as a detector comprising an array of such active pixels. The present invention also relates to a method of manufacturing such pixels and detectors.
Semiconductor based sensors and devices for detecting electromagnetic radiation are known in the art. Examples of such sensors are disclosed in WO 93/19489 and in EP-0739039. These sensors are implemented in a semiconductor substrate in CMOS- or MOS technology. In these sensors, the regions adapted for collecting charge carriers being generated by the radiation in the semiconductor substrate are forming a p-n or a n-p junction with the substrate that is of a n type conductivity or p type conductivity respectively. Such junctions are called collection junctions. Among the image sensors implemented in CMOS- or MOS-technology, CMOS or MOS image sensors with passive pixels and CMOS or MOS image sensors with active pixels are distinguished. The sensors of WO 93/19489 and EP-0739039 are active pixel sensors.
An active pixel is configured with means integrated in the pixel to amplify the charge that is collected on the light sensitive element or component in the pixel. Passive pixels do not have said means and require a charge-sensitive amplifier that is not integrated in the pixel and is connected with a long line towards the pixel. Due to the additional electronics in the active pixel, an active pixel image sensor may be equipped to execute more elaborated functions, which can be advantageous for the performance of the imaging device or system based on the sensor. Said functions can include filtering, operation at higher speed or operation in more extreme illumination conditions. It remains however a main drawback of active pixel CMOS or MOS image sensors, and to a lesser extent also of passive pixel sensors, that a significant part of the surface of the pixel is used for readout circuitry.
It is known that the charge sensitive volume of a p-n or n-p junction is larger than the depletion layer of the junction. In fact all charges generated within a so-called recombination length from the collection junction have a chance of diffusing to that junction and of being collected. Based on this mechanism it is possible to make a sensor with a small junction and yet a large photosensitive volume. Photosensors can be made with junctions of 3 by 2 xcexcm and with a recombination length of 15 xcexcm. Thus such detector has an apparent front size or photosensitive region of 30 xcexcm diameter. However if a non-related electronic circuitry such as readout circuitry is placed in the neighbourhood of such collection junction, part of the charges that otherwise would have reached the collection junction will be collected by junctions or components of the readout circuitry. The charge carriers generated by light falling on the regions of the detector that are used for readout circuitry therefore are mainly collected by the junctions of this readout circuitry. The area taken by the readout circuitry in the pixels therefore is lost for collecting the radiation and this is essentially the reason for the low fill factor or low sensitivity of active pixel based sensors.
In U.S. Pat. No. 6,225,670 a semiconductor based detector for radiation is disclosed. Such a detector is also represented in FIG. 1. It has a barrier 3 between the radiation sensitive volume 5 in the semiconductor substrate 6 and the regions 2 and junctions with readout circuitry, and no or a lower barrier 4 between the radiation sensitive volume 5 in the semiconductor substrate 6 and the regions 1 and junctions adapted and meant for collecting the charge carriers being generated by the radiation. The region forming the barrier 3 in between the radiation sensitive volume 5 wherein charges are created and the unrelated electronics 2 of the readout circuitry can have dopants of the same conductivity type as the radiation sensitive volume 5, for example a p-well in a p type substrate. The region 4 generating no barrier may be a region of inverse conductivity type as the conductivity type of the substrate, for example a n-well in a p type substrate. Such a pixel has a higher fill factor than a pixel having no barrier region 3.
It is an object of the present invention to provide sensors and methods of making the same which provide improved sharpness of image without increasing the noise content of the image.
Surprisingly it has been found by the present inventor that the fill factor of an active pixel of the kind shown in U.S. Pat. No. 6,225,670 can be increased still further by providing a distance between the region forming the barrier in between the radiation sensitive volume wherein charges are created and the electronic components of the readout circuitry on the one hand, and a region which is located at least partly under a charge collection region on the other hand.
Accordingly, the present invention provides an active pixel including a semiconductor layer having a surface and having dopants of a first conductivity type, wherein said semiconductor layer comprises a first region and a second region both having dopants of a second conductivity type, said first region and said second region being adapted for collecting charge carriers in said semiconductor layer generated by electromagnetic radiation, said first region having an area and a boundary of this area, said semiconductor layer further comprising a third region having dopants of the first conductivity type at a higher doping level than the semiconductor layer, the third region forming a barrier for substantially impeding the diffusion of said charge carriers to said second region, wherein over a part of its boundary, the first region is separated from the third region by a zone of the semiconductor layer for creation of a depletion layer or zone and a diffusion layer or zone at, or touching, the surface. The electromagnetic radiation can be all forms of light, X-rays and cosmic or nuclear particles. The semiconductor layer may be an epitaxial layer.
A fourth region may be provided having dopants of said second conductivity type and at least partially overlapping said first region, wherein the fourth region is over a part of its boundary separated from the third region by a zone of the semiconductor layer.
The present invention also provides an array of active pixels, each active pixel comprising a semiconductor layer having a surface and having dopants of a first conductivity type, wherein said semiconductor layer comprises a first region and a second region both having dopants of a second conductivity type, said first region and said second region being adapted for collecting charge carriers being generated by electromagnetic radiation in said semiconductor layer, said first region having an area and a boundary, said semiconductor layer further comprising a third region having dopants of the first conductivity type at a higher doping level than the semiconductor layer, the third region forming a barrier for substantially impeding the diffusion of said charge carriers to said second region, wherein over a part of its boundary, the first region of a pixel of interest is separated from the third region of a neighbouring pixel by a zone of the semiconductor layer for creation of a depletion layer or zone and a diffusion layer or zone at, or touching, the surface. For the best performance, the separation should at least be equal to the width of the depletion layer, which a person skilled in the art is able to calculate for a given technology. The width of this depletion layer depends on the concentration of the layer in which the depletion layer is created. Any width larger than the width of the depletion layer will not further reduce the pixel capacitance, but it might improve the sharpness. A separation smaller than the width of the depletion layer also reduces the pixel capacitance, but not as much as a separation with a width equal to the width of the depletion layer.
Within the array each active pixel can furthermore be provided with a fourth region having dopants of said second conductivity type and at least partially overlapping said first region, wherein the fourth region is over a part of its boundary separated from the third region by a zone of the semiconductor layer.
Each pixel or the array of pixels is preferably a MOS based pixel structure.
The present invention also provides a method to increase conversion gain of an active pixel including a semiconductor layer having dopants of a first conductivity type, said semiconductor layer comprising a first region and a second region both having dopants of a second conductivity type, said first region and said second region being adapted for collecting charge carriers being generated by electromagnetic radiation in said semiconductor layer, said semiconductor layer further comprising a third region having dopants of the first conductivity type at a higher doping level than the semiconductor layer, the third region forming a barrier for substantially impeding the diffusion of said charge carriers to said second region, the method comprising a step of physically separating the third region and the first region by a region of the semiconductor layer.
The present invention may also provide a method for manufacturing an active pixel comprising the steps of: providing a semiconductor layer having dopants of a first conductivity type, providing in said semiconductor layer a first region and a second region both having dopants of a second conductivity type, said first region having an area and a boundary of this area, further providing in said semiconductor layer a third region having dopants of the first conductivity type at a higher doping level than the semiconductor layer, and forming the first and third regions such that over a part of its boundary, the first region is separated from the third region by a zone of the semiconductor layer.
An active pixel according to the present invention provides a lower capacitance and a larger conversion gain than prior art active pixels. Sharper images can be produced compared with conventional devices.
Other features and advantages of the present invention will become apparent from the following detailed description, taken in conjunction with the accompanying drawings, which illustrate, by way of example, the principles of the invention. This description is given for the sake of example only, without limiting the scope of the invention. The reference figures quoted below refer to the attached drawings.